Optimized synchronization preamble structure

ABSTRACT

This proposal describes an optimized synchronization (SYNCH) symbol sequence to be used in transmission systems, which are currently under standardization. The synchronization symbol is constructed using specially designed OFDM (orthogonal frequency division multiplexing) symbols with an optimized sequence, which is mapped onto the modulated subcarriers. The resulting synchronization symbol consists of several repetitions in the time domain. Using the proposed sequence the resulting synchronization symbol achieves a high timing detection and frequency offset estimation accuracy. Furthermore the burst is optimized to achieve a very low envelope fluctuation (low Peak-to-Average Power Ratio) and a very low dynamic range to reduce complexity on the receiver and to save time and frequency acquisition time in the receiver. The proposed sequence is furthermore optimized with respect to all other synchronization symbols that are used to construct the synchronization and training preambles for the BCCH-DLCHs.

[0001] The present invention relates to a preamble structure for thesynchronization of a receiver of a OFDM transmission. The inventionfurthermore relates to an OFDM transmitter as well as to a method forthe synchronization of a receiver of an OFDM transmission system.

[0002] With reference to FIG. 2 now an autocorrelation technique on thereceiving side of an OFDM system will be explained. The received signalis delayed by a delaying unit 2 by the correlation delay D_(ac). Theconjugate complex samples of the delayed version of the signals aregenerated 3 and multiplied 4 with the received samples. The products areset into the moving average unit 6 with a window size W_(ac) and arethen postprocessed for a threshold detection and/or maximum search(units 5, 7, 8) to find the correct timing. The complex correlationresult at the peak possession generated by the unit 9 can be used toestimate the frequency offset.

[0003] A synchronization preamble structure as shown in FIG. 1 is known.This known synchronization preamble structure can be subdivided in aA-FIELD, B-FIELD and a C-FIELD. The A-FIELD and the B-FIELD aresubdivided in further parts. Each of the A-FIELD and the B-FIELD and theC-FIELD is designed to have an optimized special synchronizationfunction at the receiving side. The A-FIELD for example serves for acoarse frame detection and an automatic gain control (AGC). The B-FIELDserves as a coarse frequency offset and timing synchronization. TheC-FIELD serves for a channel estimation and fine synchronization.

[0004] Details about the concrete structure and generation of theB-FIELD can be found in the European patent application 99 103 379.6 inthe name of Sony International (Europe) GmbH, which is to be regarded asrepresenting prior art according to article 54(3) EPC. Regarding thedetails of the B-FIELD and generally the generation of the time domainsynchronization preamble signal as shown in FIG. 1 reference is made tosaid prior non-prepublished application.

[0005] The symbols of the C-FIELD, which is generally of minor interestfor the present invention, are defined in frequency domain as

C 64_(−26 . . . 26)={1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,1,1,−1,−1,1,1,−1,1,−1,1,1,1,1,0,−1,−1,1,1,−1,1,−1,1,−1,−1,−1,−1,−1,1,1,−1,−1,1,−1,1,−1,1,1,1,1}

[0006] The symbols B16 of the B-FIELD are short OFDM symbols, of whichthe subcarriers +−4, +−8, +−12, +−16, +−20, +−24 are modulated.

[0007] The content in the frequency domain is defined as:

B 16 _(−26 . . . 26)=sqrt(2)*{0,0,1+j,0,0,0,−1+j,0,0,0,−1−j,0,0,0,1−j0,0,0,01−j,0,0,0,1−j,0,0,0,0,0,0,0,0,1−j,0,0,0,−1−j,0,0,0,1−j,0,0,0,−1−j,0,0,0,0−1+j,0,0,0,1+j,0,0}

[0008] The last repetition of the B-FIELD in the time domain, calledIB16, is a sign inverted copy of the preceding B16.

[0009] The symbols A16 are short OFDM symbols, of which the subcarriers+−2, +−6, +−10, +−14, +−18, +−22, are modulated. The content in thefrequency domain is defined as:

A_(−26, . . . 26)=0,0,0,+1−j,0,0,0,+1+j,0,0,0,−1+j,0,0,0,−1−j,0,0,0,+1−j,0,0,0,−1−j,00,0,+1−j,0,0,0,−1−j,0,0,0,+1−j,0,0,0,−1−j,0,0,0,−1+j, 0,0,0,+1+j,0,0,0,0}

[0010] The sign reversal of every second A16 symbol in the time domainis automatically achieved by the specified subcarrier loading. The lastrepetition of the A-FIELD in time domain, called IA16, is a copy of thepreceding RA16.

[0011] An optimized matching between A and B-FIELD of the BCCH preambleis achieved as shown in FIG. 3 and thus the timing accuracy improvement,which is basically achieved through the specified time domain structure,is kept. Two clear single AC amplitude peaks can be identified in theBCCH preamble. Additionally a low plateau in front of the second AC peakcan be seen, which is advantageous for receiver synchronizationprocessing (e.g. used as threshold to invoke correlation peak searchalgorithm).

[0012] In the last time a new B-FIELD was proposed. In the followingthis new B-FIELD will be explained.

[0013] The symbols B16 according to this new B-field are short OFDMsymbols, of which the subcarriers +−4, +−8, +−12, +−16, +−20, +−24 aremodulated.

B 16_(−26 . . . 26)=sqrt(2)*{0,0,1+j,0,0,0,−1−j,0,0,0,1+j,0,0,0,−1−j,0,0,0−1−j,0,0,0,1+j,0,0,0,0,0,0,0−1−j,0,0,0,−1−j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0}

[0014] This new B-field results in improved performance when usingcross-correlation based receivers due to lower cross-correlationsidelobes at the border from the B-FIELD to the C-FIELD.

[0015] The short OFDM symbols, consisting of 12 modulated subcarriersare phase modulated by the elements of the symbol alphabet S={squareroot}{square root over (2)}(±1±j). The C-FIELD symbols are notconsidered here.

[0016] The generalized mapping for field B is:

S_(−26.26)=sqrt(2)*{0,0,S1,0,0,0,S2,0,0,0,S3,0,0,0,S4,0,0,0,S5,0,0,0,S6,0,0,0,0,0,0,0,S7,0,0,0,S8,0,0,0,S9,0,0,0,S10,0,0,S11,0,0,0,S12,0,0}

[0017] where ‘sqrt(2)’ is used to normalize the power. Applying a64-point IFFT to the vector S, where the remaining 15 values are set tozero ‘four’ short training symbols can be generated. The IFFT output iscyclically extended to result in the dedicated number of short symbols.

[0018] The generalized mapping for field A is:

S_(−26.26)=sqrt(2)*{0,0,0,0,S1,0,0,0,S2,0,0,0,S3,0,0,0,S4,0,0,0,S5,0,0,0,S6,0,0,0,S7,0,0,0,S8,0,0,0,S9,0,0,0,S10,0,0,0,S11,0,0,0,S12,0,0,0,0}

[0019] Where ‘sqrt(2)’ is used to normalize the power. Applying a64-point IFFT to the vector S, where the remaining 15 values are set tozero ‘four’ short training symbols can be generated. The IFFT output iscyclically extended to result in the dedicated number of short symbols.

[0020] The currently specified sequence for field A is:

S 1 . . . 12=(+1−j), (+1+j), (−1+j), (−1−j), (+1−j), (−1−j), (+1−j),(−1−j), (+1−j) (−1−j), (−1+j), (+1+j)

[0021] Using the new B-FIELD no optimization has been made in theA-FIELD in order to improve auto-correlation based receiversynchronization.

[0022]FIG. 4 shows the ideal AC result (amplitude and phase) using aBCCH preamble structure with unmodified A-FIELD, C-FIELD and the new(modified) B-FIELD based on the B16 sequence proposed. The AC result isused to identify a frame start, adjust the AGC and to do timing andfrequency synchronization. Especially the B-FIELD can be used for thelater synchronization tasks. It is very important to achieve timesynchronization as accurate as possible. With the configurationdescribed two auto-correlation peaks (A-FIELD, modified B-FIELD) arevisible, however, the slopes on both sides of the B-FIELD peak are verydifferent (steep gradient on the right, shallow gradient on the left),this effect decreases the synchronization accuracy significantly.Additionally a high plateau can be seen before the auto-correlation peakin field B (samples 105 . . . 125). This effect decreases the detectionperformance.

[0023] The above set forth latest proposed B-FIELD and A-FIELDcombination has a disadvantage that when using the new B-FIELD nooptimization has to be made in the A-FIELD in order to prove theauto-correlation properties of the corresponding receiversynchronization. The sequence to be used in the A-FIELD shouldadditionally have a minimum Peak-to-Average-Power-Ratio (PAPR) and asmall dynamic range (DR).

[0024] In view of the above disadvantages of the prior art, it is theobject of the present invention to propose A-FIELD sequences which areoptimized regarding the time domain signal properties.

[0025] It is a further object of the present invention to proposeA-FIELD sequences which are optimized regarding the resultingauto-correlation based receiver synchronization characteristics whenusing the latest proposes B-FIELD sequence.

[0026] According to a first aspect of the present invention therefore apreamble structure for the synchronization of a receiver of a OFDMtransmission system is proposed. The preamble comprises at least onefirst part. The at least one first part is designed f.e. for a coarseframe detection and/or a AGC control. The at least one first partcontains inverse fast fourier transformed frequency domain sequences ofcomplex symbols. The time domain signal of synchronization preamble isgenerated by mapping frequency domain sequences of 12 complex symbols toa 64-point IFFT according to the following scheme:

S _(−26.26)=sqrt(2)*{0,0,0,0,S1,0,0,0,S2,0,0,0,S3,0,0,0,S4,0,0,0,S5,0,0,0,S6,0,0,0,S7,0,0,0,S8,0,0,0,S9,0,0,0,S10,0,0,0,S11,0,0,0,S12,0,0,0,0},

[0027] wherein the remaining valued are set to zero.

[0028] The frequency domain sequence S_(A) of the at least one firstpart (with the appropriate A-FIELD mapping as set forth above) is one of

S 1 . . . S 12 =+A,+A,+A,+A,+A,−A,−A,+A,+A,−A,+A,−A

S 1 . . . S 12 =+A,+A,+A,+A,−A,−A,+A,+A,−A,+A,−A,+A

S 1 . . . S 12 =+A,+B,−A,−B,−A,−B,−A,−B,−A,+B,+A,−B

S 1 . . . S 12 =+A,+B,−A,−B,+A,−B,+A,−B,+A,−B,−A,+B

S 1 . . . S 12 =+A,−B,−A,+B,−A,+B,−A,+B,−A,−B,+A,+B, or

S 1 . . . S 12 =+A,−B,−A,+B,+A,+B,+A,+B,+A,+B,−A,−B

[0029] or an order reversed modification thereof.

[0030] The above sequences are also advantageous in case a preamblestructure having only one part is used as the time domain signalproperties are already improved by said sequences alone.

[0031] A second part (B-field) can be provided, wherein the frequencydomain sequence of the at least one second part corresponds to the abovecaptioned latest proposed B-field sequence, i.e.:

S _(B)=(1+j), (−1−j), (1+j), (−1−j), (−1−j), (1+j), (−1−j), (−1−j),(1+j), (1+j), (1+j), (1+j).

[0032] Particularly the A-field sequences

S 1 . . . S 12 =+A,−B,−A,+B,−A,+B,−A,+B,−A,−B,+A,+B, or

S 1 . . . S 12 =+A,−B,−A,+B,+A,+B,+A,+B,+A,+B,−A,−B

[0033] or an order reversed modification thereof, in combination withsaid B-filed sequence result in improved autocorrelationcharacteristics.

[0034] The at least one second part can follow the at least one firstpart in the time domain.

[0035] According to a further aspect of the present invention an OFDMtransmitter designed for transmitting a synchronization preamble as setforth in the BCCH channel of an OFDM system is provided.

[0036] According to a still further aspect of the present invention amethod for the synchronization of a receiver of a OFDM transmissionsystem is provided.

[0037] Further advantages, features and objects of the present inventionwill become evident for the man skilled in the art by means of thefollowing description of embodiments of the present invention taken intoconjunction with the figures of the enclosed drawings.

[0038]FIG. 1 shows the general structure of a known synchronizationpreamble,

[0039]FIG. 2 shows the general concept of an auto-correlation technique,

[0040]FIG. 3 shows a correlation result achieved with sequencesaccording to the prior art,

[0041]FIG. 4 shows an auto-correlation result achieved when using thelatest proposed B-FIELD sequence in combination with the A-FIELDsequence according to the prior art,

[0042]FIG. 5 shows the auto-correlation performance when using a firstmodified BCCH preamble according to the present invention,

[0043]FIG. 6 shows the auto-correlation performance of a modified BCCHpreamble according to another embodiment of the present invention,

[0044]FIG. 7 shows a time domain signal (power) of the known preamble,

[0045]FIG. 8 shows the time domain signal achieved by means of amodified A-FIELD according to the present invention, and

[0046]FIG. 9 shows the time domain signal (power) achieved by means of amodified A-FIELD according to another embodiment of the presentinvention.

[0047] The following sequence generation rules for the A-FIELD aresuggested which all achieve optimum PAPR and DR. Later on a subset isused which is selected with respect to optimized auto-correlationperformance in conjunction with the B-FIELD:

[0048] The use of the following A-FIELD sequences already improves thetime domain signal properties (PAPR, DR, etc.):

S 1 . . . S 12 =+A,+A,+A,+A,+A,−A,−A,+A,+A,−A,+A,−A

S 1 . . . S 12 =+A,+A,+A,+A,−A,−A,+A,+A,−A,+A,−A,+A

S 1 . . . S 12 =+A,+B,−A,−B,−A,−B,−A,−B,−A,+B,+A,−B

S 1 . . . S 12 =+A,+B,−A,−B,+A,−B,+A,−B,+A,−B,−A,+B

S 1 . . . S 12 =+A,−B,−A,+B,−A,+B,−A,+B,−A,−B,+A,+B,

S 1 . . . S 12 =+A,−B,−A,+B,+A,+B,+A,+B,+A,+B,−A,−B

[0049] with A=exp(j*2*π*φ_(A)) and B=A*exp(jπ/2)=exp(j2π*φ_(A)+jπ/2) and0.0≦φ_(A)<1.0.

[0050] More sequences can be generated by reversing the sequence order,this means replace S1 by S12, replace S2 by S11, . . . , replace S12 byS1. Note that the first two sequence kernels are binary, the rest arequaternary sequence kernels.

[0051] These sequences are advantageous also in case a preamble withonly one part is used.

[0052] The following sequences which are a subset of the above A-FIELDsequences are advantageous in combination with the latest proposedB-Filed sequence regarding the resulting autocorrelation properties:

[0053] The following first sequence is especially suitable to be used infield A (with the already explained mapping):

S 1 . . . S 12=(−1+j), (+1+j), (+1−j), (−1−j), (−1+j), (−1−j), (−1+j),(−1−j), (−1+j), (−1−j), (+1−j), (+1+j).

[0054] The following second sequence that is especially suitable to beused in field A is (with the already explained mapping):

S 1 . . . S 12=(+1−j), (−1+j), (+1−j), (−1+j), (−1+j), (+1−j), (+1−j),(−1+j), (−1+j), (−1+j), (−1+j), (−1+j).

[0055] This second sequence is especially attractive as it uses only abinary alphabet (±1)*(+1−j)

[0056] AC Performance of the Modified BCCH Preamble (First New Proposalfor the A-Field)

[0057] The negative effect shown in FIG. 4 can be avoided if the newproposed sequence is used in the A-FIELD. An optimized matching betweenA and B-FIELD of the BCCH preamble is achieved and thus the timingaccuracy improvement, which is basically achieved through the specifiedtime domain structure, is kept. Two clear single AC amplitude peaks canbe identified in the BCCH preamble if the new proposed sequence is usedfor the generation of the A-FIELD (see FIG. 5).

[0058] Furthermore, the slopes on both sides of the B-FIELD peak arevery similar (similar gradient on the right and left side of the B-FIELDauto-correlation peak), this effect increases the synchronizationaccuracy significantly. Additionally a lower plateau can be seen beforethe AC amplitude peak in field B (samples 110 . . . 130). This effectincreases the detection performance, as the plateau-value can be used asa threshold to activate a correlation-peak position detector.

[0059] One advantage of this sequence is that both auto-correlationpeaks have a very similar shape.

[0060] AC Performance of the Modified BCCH Preamble (Second New Proposalfor the A-Field)

[0061] An optimized matching between A and B-FIELD of the BCCH preambleis achieved and thus the timing accuracy improvement, which is basicallyachieved through the specified time domain structure, is kept. Two clearsingle AC amplitude peaks can be identified in the BCCH preamble if thenew proposed sequence is used for the generation of the A-FIELD (seeFIG. 6).

[0062] Furthermore, the slopes on both sides of the B-FIELD peak arevery similar (similar gradient on the right and left side of the B-FIELDauto-correlation peak), this effect increases the synchronizationaccuracy significantly. Additionally a lower plateau can be seen beforethe AC-amplitude peak in field B (samples 110 . . . 130). This effectincreases the detection performance, as the plateau-value can be used asa threshold to activate a correlation-peak position detector.

[0063] In this case the plateau is even lower as in the firstmodification and the second auto-correlation peak is very sharp.

[0064] Time Domain Signal Properties

[0065] For OFDM (or in general multicarrier signals) the signal envelopefluctuation (named Peak-to-Average-Power-Ratio=PAPR) is of greatconcern. A large PAPR result in poor transmission (due to nonlineardistortion effects of the power amplifier) and other signal limitingcomponents in the transmission system (e.g. limited dynamic range of theAD converter). For synchronization sequences it is even more desirableto have signals with a low PAPR and low dynamic range in order toaccelerate the receiver AGC (automatic gain control) locking andadjusting the reference signal value for the A/D converter (the wholedynamic range of the incoming signal should be covered by the A/Dconverter resolution without any overflow/underflow).

[0066] Currently Proposed Preamble

[0067]FIG. 7 shows the time domain power envelope of the resulting timedomain signal for the preamble. The three different fields are clearlyvisible. Field A and field B have been optimized with respect to thePAPR and DR. 8-times oversampling was considered in order to ensure thepeaks were captured correctly.

[0068] Preamble with New Proposed A-FIELD and Modified B-FIELD

[0069] The synchronization sequence design and preamble structureproposed improve the timing detection due to the jointdesign/optimization of the A-FIELD and the B-FIELD. However, PAPR and DRproperties should not be degraded.

[0070] In FIGS. 8 and 9 the two different A-FIELD options and themodified B-FIELD is used and the C-FIELD is maintained. As can be seenthere is no degradation with respect to PAPR and DR.

[0071]FIG. 8 shows the time domain signal (power) of the preamble withmodified A-FIELD (first A proposal).

[0072]FIG. 9 shows the time domain signal (power) of the preamble withmodified A-field (second A proposal).

[0073] The proposal is based on the synchronization and trainingpreambles that are already specified. Optimized sequence are proposed,which are very suitable to generate a preamble or a part (also calledfield) of it by mapping the sequence to the appropriate subcarriers ofan OFDM symbol with a IFFT size of 64. The properties of the proposedsequence with respect to PAPR and dynamic range are equal to theproperties of all currently specified sequences.

[0074] The new proposed sequences can be especially used for thegeneration of the A-field of the BCCH preamble, because this newsequence is properly matched to the specified sequence in the B-field ofthe BCCH preamble. The benefit of our proposal is the improved timingaccuracy when the AC result in the B-field of the BCCH preamble is usedfor synchronization. The time domain structures of the preambles asspecified are not touched by this proposal.

[0075] Summary of the Advantages:

[0076] An OFDM based SYNCH symbol is proposed with a reducedPeak-to-Average-Power-Ratio (PAPR)

[0077] Improved synchronization performance (timing accuracy compared tocurrent specified preamble) is achieved

[0078] No modification of the specified time domain preamble structuresis necessary

[0079] No extra complexity is needed

[0080] This proposal therefore describes an optimized synchronization(SYNCH) symbol sequence to be used in transmission systems, which arecurrently under standardization. The synchronization symbol isconstructed using specially designed OFDM (orthogonal frequency divisionmultiplexing) symbols with an optimized sequence, which is mapped ontothe modulated subcarriers. The resulting synchronization symbol consistsof several repetitions in the time domain. Using the proposed sequencethe resulting synchronization symbol achieves a high timing detectionand frequency offset estimation accuracy.

[0081] Furthermore the burst is optimized to achieve a very low envelopefluctuation (low Peak-to-Average Power Ratio) and a very low dynamicrange to reduce complexity on the receiver and to save time andfrequency acquisition time in the receiver. The proposed sequence isspecifically optimized with respect to all other synchronization symbolsthat are used to construct the synchronization and training preamblesfor the BCCH-DLCHs.

1-7. (canceled)
 8. A method for generating a synchronization preamblesignal comprising OFDM symbols, the method comprising the steps of:generating at least one OFDM symbol by modulating 12 subcarriers of anOFDM scheme according to the following sequence: S _(−26.26)=N*{0,0,(1+j),0,0,0,(−1−j),0,0,0,(1+j),0,0,0,(−1−j),0,0,0,(−1−j),0,0,0,(1+j),0,0,0,0,0,0,0,0,(−1−j),0,0,0,(−1−j),0,0,0,(1+j),0,0,0,(1+j),(1+j),00,0,(1+j),0,0} where N is a normalization factor; and inverse Fouriertransforming the generated OFDM symbol thereby generating a time domainsignal.
 9. The method according to claim 8, wherein the step of inverseFourier transforming comprises a step of applying a 64-point inversefast Fourier transform (IFFT) to the sequence S, with the remaining 15input values to the IFFT being set to zero.
 10. The method according toclaim 8, further comprising a step of cyclically extending the timedomain signal.
 11. A device for generating a synchronization preamblesignal comprising OFDM symbols, comprising: means for generating atleast one OFDM symbol by modulating 12 subcarriers of an OFDM schemeaccording to the following sequence: S _(−26.26)=N*{0,0,(1+j),0,0,0,(−1−j),0,0,0,(1+j),0,0,0,(−1−j),0,0,0,(−1−j),0,0,0,(1+j),0,0,0,0,0,0,0,(−1−j),0,0,0,(−1−j),0,0,0,(1+j),0,0,0,(1+j),(1+j),0, 0,0,(1+j),0,0} wherein N is a normalization factor; and meansfor inverse Fourier transforming the generated OFDM symbol therebygenerating a time domain signal.
 12. A method for synchronizing areceiver of an OFDM transmission system, the method comprising thefollowing steps: receiving a preamble signal; autocorrelating thereceived preamble signal, wherein the preamble signal has been obtainedby generating at least one OFDM symbol by modulating 12 subcarriers ofan OFDM scheme according to the following sequence: S _(−26.26)=N*{0,0,(1+j),0,0,0,(−1−j),0,0,0,(1+j),0,0,0,(−1−j),0,0,0,(−1−j),0,0,0,(1+j),0,0,0,0,0,0,0,(−1−j),0,0,0,(−1−j),0,0,0,(1+j),0,0,0,(1+j),(1+j),0,0,0,(1+j),0,0} where N is a normalization factor; and inverse Fouriertransforming the generated OFDM symbol thereby generating a time domainsignal.
 13. An OFDM receiver, comprising means for receiving and meansfor autocorrelating; the receiving and autocorrelating means beingdesigned for a preamble signal obtainable by the following steps:generating at least one OFDM symbol by modulating 12 subcarriers of anOFDM scheme according to the following sequence: S _(−26.26)=N*{0,0,(1+j),0,0,0,(−1 31 j),0,0,0,(1+j),0,0,0,(−1−j),0,0,0,(−1−j),0,0,0,(1+j),0,0,0,0,0,0,0,(−1−j),0,0,0,(−1−j),0,0,0,(1+j),0,0,0,(1+j),(1+j),0,0,0,(1+j),0,0} where N is a normalization factor; and inverse Fouriertransforming the generated OFDM symbol.